Plasma processes are frequently used to modify or treat the surfaces of workpieces such as semiconductor wafers, flat-panel display substrates, and lithography masks. Conditions within a plasma process are designed to produce a complex mixture of ions, reactive chemical species (free radicals), and energetic neutral species. The interaction of these materials then produces the desired effect on the surfaces of work pieces. For example, plasma processes are used to etch materials from the surfaces of semiconductor wafers so as to form complex electrical elements and circuits. The conditions within the plasma process are carefully controlled to produce the desired etch directionality and selectivities.
The surface modifications produced by a specific plasma are sensitive to a number of basic parameters within the plasma. These parameters include such variables as: chemical concentrations (partial pressures), temperatures (both surface and gas phase), and electrical parameters (ion fluxes, ion energy distribution functions). A number of these parameters (e.g. gas concentrations and pressure) can generally be easily controlled using external actuators such as Mass Flow Controllers (MFCs) and servo driven throttle valves. Other important parameters (e.g. temperatures and free radical concentrations) can often be observed or measured via sensor systems (e.g. thermocouples and Optical Emission Spectrometers (OES)) installed on the process tool. A last set of important parameters such as ion fluxes and ion energies are more difficult to either directly control or monitor.
A prime reason that these important electrical parameters are difficult to measure in a plasma process chamber is that the parameters result from a complex, nonlinear interaction between the applied driving force (RF power) and the local physical state in the process chamber. For example, a localized increase in ion concentrations can lead to locally increased RF power flows, which in turn leads to higher ion concentrations. This interaction and feedback can lead to highly non-uniform and unstable plasma conditions. It is typically impossible to adequately characterize the plasma state with a single, localized measurement.
Plasma electrical parameters have been measured with a wide variety of sensors and methods. These include: Biased probes (voltage or frequency swept), Wall probes (swept frequency), Optical emission (actinometry and Doppler), Microwave absorption, and Passive electrodes (SPORT, CHARM).
Each of these sensor types and methodologies suffer from one of more significant flaws which prevent their routine use in plasma process monitoring. Some of the most common flaws are that the sensors are either unacceptably intrusive (they excessively modify or interact with the local plasma state) or they provide an aggregate measurement lacking spatial resolution. Another deficiency found in some of the currently available techniques is their high cost due to the complexity and sensitivity of the instrumentation needed.
There is a need for improved methods and apparatuses for measuring plasma process parameters such those used for plasma processing substrates such as, but not limited to, semiconductor substrates, flat panel display substrates, and lithography mask substrates. More particularly, there is a need for improved methods and apparatuses for measuring process parameters such as plasma density, plasma uniformity, ion energy distributions, electron energy distributions, ion fluxes, and ion energies.